1. Field of the Invention
The present invention relates generally to the fields of de-watering sludge. More particularly, it concerns methods and apparatus for de-watering sludge by utilizing a multi-stage, rotating drum technique.
2. Description of Related Art
The ability to remove water from products such as waste products has long been recognized as useful. Removing water from sludge, and more particularly, sewage sludge, greatly reduces the weight of material to be transported for disposal and/or use. Additionally, de-watering sludge may facilitate the processing of sludge into soil conditioners and fertilizers.
De-watering of sludge has been accomplished in the past by various methods, including open-air drying, vacuum filtration, centrifugation, mechanical pressing, and other mechanical separation. Although each has utilized at least a degree of usefulness, room for significant improvement remains.
U.S. Pat. No. 3,695,173 describes a technique for sludge de-watering. There, de-watering is achieved as sludge is moved through a filter de-watering unit, where squeezing and pressing takes place. The squeezing and pressing forces most of the liquid or water through a filtering medium or slots. Cleaning blades or brushes are used prevent clogging and interruption of continuous filtration or de-watering. Although this technique may be useful for de-watering certain types of sludge, problems remain. For instance, if there is too little sludge being fed into the device, the sludge may not be squeezed enough. Consequently it may not be dried sufficiently. Further, if too much sludge enters the device, it may come under too much pressure and may be forced out through the device. Consequently, sludge may be lost and may need to be re-circulated. Still further, if a polymer has been used on the sludge being de-watered, this system may, if subjected to high pressures, break polymer bonds and may therefore not be effective in de-watering sewage sludge, especially recirculated sludge.
U.S. Pat. No. 4,286,512 describes another technique that may be used to remove liquid from a material. There, a screw press is used for pressing liquid from fibrous slurries, such as paper pulp, sludge, or sedimentation. Material is fed into the press at one end of a rotating press screw. The core of the screw has an increasing diameter such that the space defined between the core and wall of a drum gradually decreases so as to aid in the pressing process. A rotating drum rotates about the core to further aid in the draining of liquid pressed from the slurry due to the action of the press-screw. Although this technique also may be useful for de-watering certain types of sludge, similar problems remain due mostly to the fact that this technique relies upon pressing of the sludge to achieve drying. Again, if there is too little sludge being fed into the device, the sludge may not be squeezed enough. On the other hand, if too much sludge enters the device, it may come under too much pressure and may be prematurely forced out of the device. Consequently, sludge may be lost and may need to be re-circulated. Still further, if a polymer has been used on the sludge being de-watered, this system may, if subjected to high pressures, break polymer bonds. For this reason, such a system may not be effective in de-watering sewage sludge, especially re-circulated sludge.
U.S. Pat. No. 4,202,773 describes another technique that may be used to de-water sludge. There, clarifier sludge may be de-watered by passing it through an inner centrifuge drum and then to outer second drum having a diameter about twice that of the first drum. In each drum, wipers or scrapers displace solids. Each drum is unperforated, the outer drum is conical over its entire length, and the inner drum is conical in part or in whole. Although this system may be useful for de-watering certain materials, problems remain. For instance, it appears that sludge is spun at relatively high speeds to insure that solids within the sludge are displaced and so that sludge does not overly mix with water and liquids being extracted. Further, it appears that this device may subject sludge to great forces, particularly after transfer to the larger outer centrifuge, so that polymer bonds may be broken.
Problems enumerated above are not intended to be exhaustive but rather are among several that tend to impair the effectiveness of previously known devices for removing water and liquids from sludge. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that devices appearing in the art have not been altogether satisfactory.